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Amniotes developed the cleidoic egg early in the Carboniferous period, and this meant they could live and reproduce on dry land. These early land animals evolved into the reptiles, birds and mammals of today. In contrast, the amphibia are land animals who must lay their eggs in water, because the eggs are not protected by a shell.

The structure and composition of the eggshell protects the egg against damage, microbial contamination, and desiccation (drying out). Small molecules can pass through it, so the shell allows gas exchange for the growing embryo. It also provides calcium for the embryo's skeleton.

While many reptiles lay eggs with flexible, calcified, eggshells, there are some that lay hard eggs. Eggs laid by snakes generally have leathery shells which often adhere to one another. Depending on the species, turtles and tortoises lay hard or soft eggs. Several species lay eggs which are nearly the same as bird eggs.

The normal bird egg is a fertilized egg sitting on the yolk surface and surrounded by albumen, or egg white. The albumen in turn is surrounded by two shell membranes (inner and outer membranes) and then the eggshell. The chicken eggshell is 95-97% calcium carbonate crystals, which are held together by a protein matrix.[1][2][3]

While the bulk of eggshell is made of calcium carbonate, it is now thought that the protein matrix has an important role to play in eggshell strength.[4] These proteins affect crystallization, which in turn affects the eggshell structure.

The average laying hen, takes about 20 hours to make the shell. Pigmentation is added to the shell by papillae lining the oviduct, coloring it any of a variety of colors and patterns depending on the species.